Determination of 4.438 MeV γ-ray to neutron emission ratio from a 241Am–9Be neutron source

doi:10.1016/j.apradiso.2004.02.008

Applied Radiation and Isotopes

Volume 60, Issue 6, June 2004, Pages 959-962

https://doi.org/10.1016/j.apradiso.2004.02.008 Get rights and content

Abstract

Gamma-ray (γ-ray) spectra of a 1.49×10¹¹ Bq ²⁴¹Am–⁹Be source and background were measured using a 2 in×2 in, NaI(Tl) detector. Backgrounds due to the neutron interactions and energy deposition were calculated with MCNP4C. By subtracting the backgrounds from the experimental spectra, the S_γ is obtained and the R=S_γ/S_n was estimated. The final result of R=S_γ/S_n=0.596 is in agreement with result reported in literature.

Introduction

All (α,n) neutron sources are made from α-emitting isotope and suitably low-Z targets. ⁹Be is the most important target because it offers the highest neutron yield. A stable alloy can be formed between Be and actinide α-emitters of the form MBe₁₃, where M represents the actinide metal (Knoll, 2000). The (α,n) neutron sources are used for many application like activation analysis (Pinault, 1998; Shahriari and Sohrabpour, 2000), calibration source (Croft, 1989), and industrial applications (Akaho et al., 2001; Jonah et al., 1992), because they are relatively inexpensive, compact, portable, and reasonably constant although they yield a rather low output.

²⁴¹Am–⁹Be source has a long half-life period (432.7 yr) and is therefore used in many laboratories especially as a neutron and gamma calibration source (Norman, 2001). It is not only a common neutron source but also a γ source that produces 4.438 MeV photons.

Neutron and photon reaction channels are, $α+^{9} Be →^{13} C^{*} → n_{0} +^{12} C +γ_{1} [Q=5.701 MeV], (a) n_{1} +^{12} C +γ_{1} [Q=1.262 MeV, E_{γ_{1}} =4.438 MeV], (b) n_{2} +^{12} C +γ_{1} +γ_{2} [E_{th} =2.821 MeV, E_{γ_{1}} =3.216 MeV], (c) n_{3} +^{12} C +γ_{3} [E_{th} =5.691 MeV, E_{γ_{3}} =9.641 MeV] . (d)$

There are also other channels for this reaction but their probabilities are very low. Fig. 1 shows the energy level and the electromagnetic transition of ¹²C. ⁹Be(α,n)¹²C reaction in ²⁴¹Am–⁹Be source, almost leads to either ground state or first-level excited state, [channels (a) and (b)] (Norman, 2001; Croft, 1989). The intensity of 4.438 MeV γ-ray to neutron ratio, R=S_γ/S_n, is a very important characteristic of (α,n) sources, specially in mixed n–γ field spectroscopy (Pal et al., 1998) and dosimetry (Milman et al., 2001).

In the present work, the γ-ray pulse height spectrum of a ²⁴¹Am–⁹Be source was measured using a NaI(Tl) detector. The pulse height spectrum was calculated using the MCNP4C code (Briesmeister, 2000) and the ratio of 4.438 MeV γ-ray to neutron was calculated.

Section snippets

Measurements

The gamma pulse height spectrum of a ²⁴¹Am–⁹Be source with 1.49×10¹¹ Bq activity and background were measured by a cylinderical, 2″×2″, NaI(Tl) detector. Measuring time was 10 min, both spectra are shown in Fig. 2. These spectra are in agreement with those measured by Vega-Carrillo et al. (2002). The source was located 110±0.5 cm above the floor level, 78±0.5 cm from the center of the detector, and 165±0.5 cm from the nearest wall. In the measured pulse height spectrum the following peaks can be

Background and contamination in the ²⁴¹Am–⁹Be pulse height spectrum

Due to cosmic radiation that continuously bombard the earth's atmosphere, the existence of natural radioactivity in the environment, and the presence of radioactive sources in the lab, all detectors record some background signal. Also, in gamma spectroscopy in mixed γ–n field, due to the interaction of neutrons, there is an on-line background.

So, the ²⁴¹Am–⁹Be pulse height spectrum is contaminated by

•
The normal background is due to sources present in the lab and the natural γ-ray emitters

Computations and discussion

To obtain the net spectrum of 4.438 MeV γ-ray, the three kinds of spectrum contaminations were subtracted from the experimental spectrum. Detector efficiency and pulse height spectrum were calculated using a 4.438 MeV gamma source instead of the neutron source. Then we normalized the smoothened Monte Carlo pulse height spectrum with the highest peak (3.927 MeV single scape peak) of the net spectrum. Fig. 4 shows a good agreement between them, so we can accept the total net counts, N_t integral of

Conclusion

The ratio of the 4.438 MeV γ-ray to neutron in the ²⁴¹Am–⁹Be source was calculated using the experimental–computational approach. The correction of gamma spectrum due to the interaction of neutrons with the materials is possible only with a Monte Carlo simulation, although a low scatter room can reduce the effect. The present work demonstrates a useful approach using MCNP code that can be applied in many other fields.

Acknowledgements

The authors would like to thank Prof. G. Furlan, head of the TRIL program at ICTP, Trieste, Italy, for his contribution in this work.

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Determination of 4.438 MeV γ-ray to neutron emission ratio from a 241Am–9Be neutron source

Abstract

Introduction

Section snippets

Measurements

Background and contamination in the 241Am–9Be pulse height spectrum

Computations and discussion

Conclusion

Acknowledgements

Geometrical effects on thermal neutron reaction of hydrogenous moderators using 241Am–Be source

Appl. Radiat. Isot.

The use of neutron intensity calibration 9Be(α,n) sources as 4438 keV gamma-ray reference standards

Nucl. Instrum. Methods Phys. Res. A

An improved neutron reaction setup for the determination of H and (O+C)/H in oil samples

J. Radioanal. Nucl. Chem.

Radiation Detection and Measurement

Dosimetry of mixed gamma-neutron fields using TLD-500 K detectors based anion-defective corundum

Radiat. Meas.

Determination of 4.438 MeV γ-ray to neutron emission ratio from a ²⁴¹Am–⁹Be neutron source

Background and contamination in the ²⁴¹Am–⁹Be pulse height spectrum

Geometrical effects on thermal neutron reaction of hydrogenous moderators using ²⁴¹Am–Be source

The use of neutron intensity calibration ⁹Be(α,n) sources as 4438 keV gamma-ray reference standards